Added CONTRIBUTING.md

CONTRIBUTING.md

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+Contributing
+============
+
+Code contributions are most welcome and highly appreciated!
+
+But first, please note that although bsnes is licensed under the GPLv3 license,
+in order to be merged upstream, any code contributions must be provided under
+the ISC source code license.
+
+This is *not* a CLA (community license agreement), no legal contract needs to be
+signed, and you will maintain full and exclusive copyright ownership over any
+contributed source code.
+
+There are two reasons for this requirement:
+
+GPLv4+
+------
+
+bsnes is currently licensed under the GPLv3 license only. I do not license bsnes
+under the GPLv3 or later license, because there is no way of knowing what the
+GPLv4 and later licenses will change, and if they will be in the best interests
+of emulator development and video game preservation.
+
+Although I put a good deal of trust into the FSF, no one is an oracle that can
+predict the future. Would /you/ agree to a license before being able to read it?
+
+However, the GPLv4 may prove beneficial, and close important holes in the GPLv3
+license, just as the GPLv3 license closed the GPLv2's TiVoization loophole. And
+so it is important that bsnes retains the option of relicensing to the GPLv4+ in
+the future.
+
+As a point of interest, there have been projects with similar concerns about
+using a GPLv2 or later clause, that are now permanently stuck on the GPLv2
+license. There have also been projects that did use a GPLv2 or later clause,
+only to disagree with the changes introduced in the GPLv3.
+
+ISC
+---
+
+The more important reason for this requirement is that it is my intention to
+release the entirety of bsnes under the ISC license once official upstream
+development has ceased.
+
+The reason I would want to relicense bsnes to the ISC license upon its official
+discontinuation is because once again, no one is an oracle, and I cannot predict
+what future issues bsnes permanently remaining under the GPLv3 license may
+cause.
+
+For instance, imagine a world where a certain vendor took over the world, and
+the only way to distribute applications was with their approval, and their store
+rules forbade GPLv3 software. Or perhaps a world where the GPL was abandoned in
+favor of the new OSSv1 license. But GPLv3 software was incompatible with the
+OSSv1 license. Other open source developers would not be able to use bsnes in
+that scenario.
+
+It would be very disappointing if all of our work ended up unusable 50+ years
+into the future because it was permanently bound to the GPLv3 license.
+
+GPLv3
+-----
+
+The reason I use the GPLv3 license currently is because it is a balance between
+altruism and self-interest. The GPLv3 allows other vendors to sell my own code
+without sharing revenue with me, and indeed this has already happened. But the
+GPLv3 also prevents other vendors from improving upon bsnes without sharing
+their work with everyone else as I have.
+
+While I am actively developing bsnes, I do not wish to compete against myself.
+
+As such, I believe the GPLv3 is the best license during active development, and
+the ISC is the best license once bsnes is officially discontinued.
+
+Considerations
+--------------
+
+This is the part that should concern you as a contributor: I am not requesting
+contributed source code to be released under the ISC so that I personally may
+sell GPLv3 commercial license exemptions to your work, but in the future when
+bsnes is released under the ISC license, that will open the door for anyone to
+sell the work commercially in a closed source form.
+
+If this is not acceptable to you, I wholly understand and I welcome you to
+release your work under the GPLv3 in the form of a bsnes fork. And if your work
+is not an essential part of the core emulation -- that is to say, it may be
+optionally disabled -- then I am still willing to work with you in merging such
+work upstream anyway under the full GPLv3 license, but please reach out to me
+first before developing under the assumption your work will be merged upstream.
+
+Thank you very much for reading and hopefully for your understanding.